For many applications, Radio Frequency (RF) Power Amplifiers (PAs) are required to have an output signal power that is independent of PA supply voltage and PA operating temperature.
Prior art PA circuits generally have a first stage that is supplied with a ramped voltage supply that is used to control an output signal of the first stage in a controllable manner. Unfortunately, a disadvantage of the old technology is that fine control for controlling of amplitude of the output signal is not easily attained. A collector emitter saturation voltage of the first stage RF transistor, or transistors, limits the first stage output signal. At saturation, the amplitude of the output signal is hard to predict with any great accuracy. As a result, this does not provide an overall PA output signal amplitude with accuracy. This poses a particular problem when the supply voltage is very low in potential and the variability of the saturation of the PA amplification stages significantly alters the power output capability. As a result, a closely controlled PA output signal level is not readily obtained. Typically, the maximum output signal power of the PA is proportional to the supply voltage or a significant efficiency penalty is suffered. Having the output signal power vary upon power supply variations is unacceptable for many applications.
In cellular telephone handsets and other wireless applications, because of battery power there is provided to circuitry therein a particularly wide range of supply voltages. As a result, there is a considerable difference between the PA supply voltage when a cellular telephone handset battery is fully charged and significantly discharged.
A need therefore exists to provide circuitry that during operation thereof provides a RF output signal that is approximately independent of the actual supply voltage potential. It is therefore an object of the invention to provide an input stage for an integrated PA with a more accurately controllable output signal power than is currently attainable in the state of the art.